The present invention relates to a sliding bearing for an internal combustion engine, and more particularly to a sliding bearing consisting of a copper-based bearing alloy, on which an overlay is applied.
The present applicant proposed in (1) Japanese Unexamined Patent Publication No. 9-249,924 and (2) European patent publication No. 0795693A2 a copper alloy, which has a particular structure considerably exceeding the properties of the kelmet used heretofore as the sliding-bearing alloy of an internal combustion engine. The alloy proposed in publication (1) is a copper alloy, which contains As; Sn, Sb, In, Mn, Fe, Bi, Zn, Ni and/or Cr as the solute element(s) of a Cu matrix and, further, essentially no secondary phase consisting of or containing these elements is formed. Likewise, the surface layer of the sliding-bearing alloy proposed in (2) is exposed when an overlay is locally worn out during the initial breaking-in of the overlay. At least a portion of the exposed surface layer consists of a copper alloy in which the above-mentioned elements such as Ag and the like are concentrated. At least the boundary of the bulk portion contiguous to the copper-alloy surface and its vicinity contain the above-mentioned elements such as Ag and the like in the solid solution and consists of such solid solution is essentially free from a secondary phase consisting of or containing these elements.
The sliding bearing proposed in the above-mentioned publication (2) consists of a copper alloy, which contains Ag, Sn, Sb, In, Al, Mg and/or Cd, and Cu essentially in balance, and which is bonded to the backing metal. Ag and the like are solid-dissolved in the Cu matrix at least in the vicinity of the sliding surface. Essentially no secondary phase, such as an Ag phase, is formed. A phase, which contains a hexagonal compound of Ag and the like with one another or Ag and the like with Cu, a compound of the Ag and the like with sulfur and oxygen, or an eutectic, is formed on the surface caused to slide with an opposing shaft.
An overlay is unnecessary or an extremely thin overlay is sufficient for the sliding bearings proposed in these publications (1) and (2), because the seizure resistance of the copper alloys in these publication is improved.
Incidentally, when a sliding bearing is used under high surface pressure, the shaft deflects by a few microns, with the result that the localized surface pressure of the bearing becomes so high that seizure is liable to occur at such portions. The life of a sliding is therefore limited from the aspect of surface pressure. In the most general kelmet bearing (thickness of lining=0.2 mm, Ni barrier=2 xcexcm, Pb-based overlay=20 xcexcm) the life of such kelmet is a million km under surface pressure of 7 MPa. Surface pressure of 70 MPa corresponds to an engine with 4000-8000 cc of displacement, equipped with a turbo-charger.
It is expected that the sliding-bearing alloys proposed in the above-mentioned publications (1) and (2) exceed the surface pressure mentioned above. However, the above mentioned publications (1) and (2) give no consideration as to which overlay is optimum for a sliding bearing used under high surface pressure. The present inventors tested, therefore, various overlays and carried out research for the purpose of providing a sliding bearing for an internal combustion engine capable of being used under higher load than heretofore.
The sliding bearing of an internal combustion engine according to the present invention is characterized in that: a copper alloy contains from 0.1 to 2% by weight of Ag and from 1 to 10% by weight of Sn as the essential elements, the balance essentially consisting of Cu, is bonded to a backing metal, and has on its side opposite to the backing metal a roughened surface of approximately 0.5 to approximately 10 xcexcm of roughness (Rz); the roughened surface is coated with at least one thermo-setting resin, which is selected from the group consisting of polyimide resin, polyamide-imide resin, epoxy resin and phenol resin, and which contains from 55 to 95% by weight of MoS2; Ag and Sn are solid-dissolved in the Cu matrix of the copper alloy in at least the vicinity of the sliding surface where essentially no secondary phase of these elements is formed; and, a concentrated layer of said Ag and Sn, a hexagonal compound of these Ag and Sn with one another, a hexagonal compound of Cu and these elements, or a eutectic of Ag and Sn or Cu and these elements, is formed as a sub-layer of at least a portion of the sliding layer, which portion is brought into direct contact with an opposing shaft.
In addition, according to an embodiment of the sliding bearing, there is provided a sliding bearing for an internal combustion engine: wherein its copper alloy contains 10% by weight or less of at least one additive element selected from the group consisting of Ab, In, Al, Mg and Cd; the essential elements and the additive elements are solid-dissolved in the Cu matrix of the copper alloy in at least the vicinity of the sliding surface where essentially no secondary phase of these elements is formed; and, a concentrated layer of said essential and additive elements, a hexagonal compound of these elements with one another, a hexagonal compound of Cu and these elements, or a eutectic of said essential elements and additive elements or Cu and these elements, is formed as a sub-layer of at least a portion of the sliding layer, which portion is brought into direct contact with an opposing shaft.
The present invention is described hereinafter in detail.
First, the copper alloy used in the present invention is explained. This copper alloy is based on the publications (1) and (2) by the present applicant mentioned above. Specifically, the following points are utilized. The particular additive elements, which are solid-dissolved in the Cu matrix, move to the lining surface, while friction heat generates and the structure of the lining surface changes. The particular additive elements then locally form a concentrated layer. A hexagonal compound or a eutectic composition, which is formed as the concentration progresses to some extent, has excellent solid-lubrication effect and excellent sliding performance under high surface pressure.
In basic experiments the seizure resistance of various compounds was investigated. The results are hereinafter explained.
A metal sheet or an alloy sheet, the composition of which is shown in Table 1, was cast or rolled and heat-treated to form a hexagonal compound shown in the equilibrium phase-diagram. However, the heat treatment was not carried out for No. 3 having a eutectic composition. The sheet was then worked in the form of a specimen (1 cm2 of the surface area, 1.0-1.5 xcexcm of roughness Rz). The specimens were subjected to a test of seizure resistance under the following conditions.
Tester: a pin-on disc tester shown in FIG. 2
Sliding Speed: 15 m/s
Load: Gradual increase of load (step mode), 500N/10 min
Kind of oil: 10 w-30
Temperature of oil: room temperature
Opposed material: hardened S55C (Hv 550-650),
roughnessxe2x80x940.5-0.8 xcexcm Rz
In FIG. 2: 5xe2x80x94oil-feeding pad; 6xe2x80x94hydraulic cylinder; 7xe2x80x94a test piece; 8xe2x80x94disc; 9xe2x80x94balance weight; and 10xe2x80x94a load cell.
The results are shown in Table 1.
As is apparent from Table 1, the compounds or eutectic of Nos. 1-15 have seizure resistance approximately 1.5 times or more as high as that of the pure metal such as Cu, Ag or Sn. Although the seizure resistance of metallic Ag (No. 17) and the metallic Sn (No. 18) are poor, No. 3 (eutectic), in which these metals are finely mixed, has high seizure resistance. An effect due to the coexisting different elements is thus recognized. Enhancement of seizure resistance due to the hexagonal compound is believed to be due to the co-presence effect of the different elements, and cleavage of the hexagonal compound. MoS2, graphite, hxe2x80x94BN and the like having a hcp structure, have cleavage property and thus low-friction property, with the result that the seizure resistance is enhanced. This fact would similarly explain how the seizure resistance is enhanced in the present invention by the hexagonal compound.
Materials including No. 1 (h-Ag3Sn), No. 3 (AgSn eutectic), and No. 10 (Cuxe2x80x94Sn eutectic) of Table 1 were subjected to a basic test for measurement of the friction coefficient and adhesion resistance. The test was carried out under the following conditions.
Tester: A Bouden/Teber stick-slip tester shown in FIG. 3
Sliding Speed: 0.06 m/s
Load: 5N
Lubricating Condition: application of oil
Opposed Material: SUJ 2 (8 mm in diameter)
In FIG. 3: 11xe2x80x94pin; 12xe2x80x94test specimen: and 13xe2x80x94heater
The results are shown in Table 2.
From Table 2, it is revealed that No. 1 of the hexagonal compound is the most resistant to adhesion. Pure Ag (No. 17) is the second most resistant to adhesion. The adhesion resistance of the eutectic (No. 3) and the hexagonal compound (No. 1) is excellent. The resistance to adhesion of pure Sn is the lowest. The resistance to adhesion of pure Cu is the worst.
Based on the results of the basic experiments, Ag and Sn turned out to be the most effective elements for forming on the surface of a lining a hexagonal compound(s) or eutectic and hence enhancing the seizure resistance
The present inventors further advanced the research and discovered that it is important to once solid-dissolve the Ag, Sn or the like in the lining before use, that is, these additive elements should not form the secondary phase before use. More specifically, no secondary phase should be identified, under the X-ray diffraction condition described hereinbelow, in the alloy""s surface portion participating in the sliding. When a secondary phase is formed, no matter whether the solute element(s) of the copper-alloy matrix is in an equilibrium state or non-equilibrium state, the concentration of the additive element(s) in the lining surface after sliding is difficult.
The common properties of the above additive elements, such as Ag, Sn, Cd, In, Mg, Sb and Al are as follows. (a) They are liable to be alloyed with copper and do not considerably harden copper. (b) They are highly resistant to deteriorated lubricating oil. (c) They are liable to concentrate on the lining surface. (d) They exhibit improved friction coefficient, corrosion resistance, non-adhesiveness and the like under the co-presence of different kinds of elements. (e) They are soluble in the solid solution. (f) They do not easily precipitate. (g) They form a hexagonal compound or a eutectic.
Other elements than the above-mentioned ones, for example Ca and Na, do not fulfill (a). Pb cannot be employed in the light of (b). V and W having a large mass are difficult to diffuse in the copper alloy and therefore do not satisfy (c). Pb and Bi have such large difference in the melting point from that of Cu that it is difficult to minimize the phase separation during casting. Pb and Bi, therefore, do not satisfy (e).
The following points are necessary in the present invention. That is, the solid-solution state of the additive element(s) is maintained in the copper alloy during a certain period of use of the bearing. As the friction progresses, solid contact between the shaft and the lining occurs frequently. When such a condition arises, the additive element(s) concentrate on the sliding surface of the lining. Since an element, which is liable to precipitate, easily forms a secondary phase, the amount of the solute element as a source of the concentrated layer is disadvantageously diminished. The condition (f) is, therefore, also important, and the additive elements of the known precipitation-type alloys are excluded from the present invention.
The hexagonal compound mentioned in (g) is, for example, an Agxe2x80x94Sn compound (xcex6-zeta phase). This hexagonal compound is formed when Ag and Sn are present on the surface of a bearing in a weight ratio of Ag: Sn of 85 to 15 or its vicinity, and exceed their solubility in Cu, and further, energy is imparted to form a compound. This energy is the bearing temperature under the normal sliding condition of a bearing being used, for example 120xc2x0 C. or more in terms of the oil temperature. Note that Ag and Sn are solid-dissolved in the Cu alloy matrix, preferably in super-saturation; and, further, no secondary phases of these elements are formed in the matrix. Alternatively, a bearing can be subjected, prior to use, to the same conditions as mentioned above. That is, a bearing is exposed to pressure equivalent to that from a shaft and to heat equivalent to the oil temperature, so as to impart a temperature gradient equivalent to that during use. These elements thus concentrate on the surface of a bearing. Although the sliding performance can be enhanced even under such a concentration state, it can be further enhanced when these elements partly form a hexagonal compound. Evidently, the sliding performance can be further enhanced with an increase in the proportion of the hexagonal compound.
The eutectic mentioned in item (g) is basically the same as the hexagonal compound. Noteworthy points of the eutectic are explained with regard to a Cuxe2x80x94Agxe2x80x94Sn system having an eutectic at 3.5% by weight of Ag and 96.5% of Sn, between Sn and Ag3Sn which is an intermetallic compound having a relatively low melting point. When Ag and Sn, are once (super-saturation) dissolved in the Cuxe2x80x94Agxe2x80x94Sn alloy, then concentrated on the surface layer of a bearing, such a structure is formed. That is, Ag3Sn and Sn disperse finely and thinly on the surface of a Cu matrix. As a result, the seizure resistance is greatly enhanced as compared with that attained by concentration of a single element, as shown in Tables 1 and 2.
The equilibrium solid-solution amount of the above-mentioned elements in a binary alloy is determined by a phase diagram (M. Hansen, Constitution of Binary Alloys, McGraw Hill Company, New York, 1964), for example, 0.1% by weight for Ag, 1.3% by weight for Sn, and 0.5% by weight for Cd at approximately 200xc2x0 C. In a ternary alloy, practical determination of the equilibrium solid-solution amount can be made such that, when any one of the additive elements exceeds the equilibrium solubility of a binary alloy, the solute elements in a non-equilibrium state are contained in such a ternary alloy.
The copper alloy, which contains the additive element(s) in a non-equilibrium state, is produced preferably by the casting method or atomizing method. In the casting method, the melt is cooled at a cooling speed of 100xc2x0 C./minute or higher, which forces the additive element(s) to dissolve in the Cu matrix. The working steps may be carried out subsequent to the casting in such a manner that the dissolved element(s) do not precipitate. However, since such working step must be carried out very carefully, a continuously cast strip is preferably used as a lining as it is. In the case of the sintering method, the atomized powder, which is produced by high-speed cooling of the copper-alloy melt, is then subjected to sintering at a solution temperature of the additive element(s), followed by rapid cooling at a cooling speed of 50xc2x0 C./minute or higher. In addition to the above additive element(s), from 0.01 to 0.5% of P can be added as a de-oxidizing agent or a sinter-promoting agent.
The balance of the above composition is impurities ordinarily contained in the copper, such as Si, O and the like. The purity of copper may be such as that of tough-pitch copper, electric copper, electrolytically refined copper, and OFHC (Oxygen Free High Conductivity Copper). S, which is permissible as an impurity, has virtually no solubility in Cu and is, therefore, present as the Cuxe2x80x94S secondary phase.
When the copper alloy according to the present invention is rendered in the form of sintered material, the sintered pores may be impregnated with resin. Almost all resins used as the sliding material can be used as the impregnating resin. They are preferably PI, PAI, PEI, PEEK, aromatic PA, phenol resin, epoxy resin, PTFE and, in addition, fluorine resin (PFA, ETFE and FEP). The amount of resin is preferably from 30 to 80% by volume, more preferably from 40 to 60% by volume. Porosity of the sintered material is preferably from 70 to 20% by volume, more preferably from 60 to 40% by volume. When PTFE and other fluorine resin are used, the porosity of sintered material may be small, for example from 60 to 20%.
A solid lubricant, wear-resistant additive or the like can be mixed in the impregnating resin. Specifically, the solid lubricant is such as graphite, PTFF, Pb, Pb-Sn alloy, carbon fluoride, and lead fluoride. The wear-resistant additives are Al2O3, SiO2, Si3N4, clay, talc, TiO2, mulite, calcium carbide, Zn, AlN, Fe3P, Fe2B, Ni2B, FeB, and spheroidal carbon. In addition, inorganic fiber, such as glass fiber, carbon fiber, potassium titanate fiber and the like can be used. Organic fiber, such as aromatic PA fiber, whisker such as SiC whisker, and metal fiber such as Cu fiber, stainless steel fiber and the like can also be used.
The rolled or sintered copper alloy for a sliding bearing may be bonded on the metal backing to provide a sliding bearing, or may also be in the form of a solid bearing not bonded on the metal backing.
The copper alloy according to the present invention is used for various bearings for an engine, a connecting-rod bearing and other bearings of an internal combustion engine.
When the surface of the lining is analyzed by SIMS (Secondary Ion Mass Spectroscopy) method after sliding, concentrated regions of the additive element(s) are detected. Such concentrated layer is very thin, of thinness of 1 xcexcm or less. The concentration of the additive element(s) in the concentrated layer is, for example, 1.3 times or more as high as that in the alloy bulk. A part of the concentrated elements is converted to a hexagonal compound or a eutectic. The hexagonal compound or eutectic is liable to form in a region where the concentration ratio is twice or higher. When the sliding further advances, the concentrated layer reacts with sulfur in the lubricating oil, with the result that the seizure resistance is enhanced.
The base oil of the lubricating oil and its additives, which are used in the present invention, is not limited at all. The sulfur-based additives, which may be used as an additive, are such compounds as (poly)sulfide, sulfonate, sulfinate, sulfenate, the phenate having the structure formula given below, (di)thiophsphate compound, thioketone, thioacetal, thiocarbonic acid and its derivative(s), sulfoxide and its derivative(s), sulfonyl, sulfinyl, sulfenyl, and ZnDTP. Each of these organic-acid compounds decomposes at 100-160xc2x0 C., which is the sliding temperature of a sliding bearing, into a reactive sulfuric-acid based acid which is then caused to react with the concentrates on the surface of the copper alloy.